Transistor Circuit Substitute for SCR

Figure 1.  A Transistor-based Circuit Equivalent to an SCR

A silicon-controlled rectifier (SCR) is a solid-state device that doesn't allow current to flow until it is triggered to conduct. The SCR is a four-layer device, i.e., its structure consists of four layers of alternating semiconductor material type: p-n-p-n.  Once triggered, it will conduct current in only one direction. SCR's are generally used for AC switching. 

The transistor circuit shown in Figure 1 is equivalent to a silicon-controlled rectifier (SCR).  It employs a PNP and an NPN bipolar transistor to mimic the p-n-p-n structure of an SCR.  In this circuit, once NPN transistor Q2 is triggered by a positive input to its base, it starts pulling down the voltage at the base of PNP transistor Q1.  This would drive Q1 to conduct, which will then supply Q2's base with continuous current to keep it 'on'.

Under this state, both transistors will conduct indefinitely, as long as there is available current flowing between the anode (A) and the cathode (K).  The circuit will only turn 'off' if the supply of current from A to K is interrupted.